1. Field of the Invention
The present invention relates to a drive circuit for alternately turning on first and second semiconductor switching elements that are connected in series.
2. Description of the Related Art
FIG. 1 illustrates an example of a drive circuit for driving semiconductor switching elements according to a related art. In FIG. 1, a first switching element Q1 of an n-type MOSFET for high side and a second switching element Q2 of an n-type MOSFET for low side are connected in series. Both ends of this series circuit are connected to a DC power source VDC. A connection point of the first and second switching elements Q1 and Q2 is connected to a load 1.
The first and second switching elements Q1 and Q2 are controlled according to a control signal, i.e., a pulse signal generated by a pulse signal generator 2. Both ends of the pulse signal generator 2 are connected to a series circuit including a capacitor C and a primary winding P of a transformer T1. The transformer T1 has the primary winding P, a first secondary winding S1, and a second secondary winding S2. The first secondary winding S1 and primary winding P are oppositely wound and the second secondary winding S2 and primary winding P are wound in the same direction.
The first secondary winding S1 of the transformer T1 is connected to a first driver 3c. Based on a voltage provided by the first secondary winding S1, the first driver 3c applies a first drive signal to a gate of the first switching element Q1. The second secondary winding S2 of the transformer T1 is connected to a second driver 3d. Based on a voltage provided by the second secondary winding S2, the second driver 3d applies a second drive signal to a gate of the second switching element Q2.
Operation of the drive circuit illustrated in FIG. 1 will be explained with reference to a waveform diagram of FIG. 2.
At time t0, the pulse signal generator 2 generates a pulse signal PL, which is transferred through the capacitor C and the primary winding P of the transformer T1 to the first and second secondary windings S1 and S2. The first and second secondary windings S1 and S2 are oppositely wound, and therefore, pulse signals that are inverted from each other are sent to the first and second drivers 3c and 3d. The first driver 3c applies the first drive signal Hg to the gate of the first switching element Q1 and the second driver 3d applies the second drive signal Lg to the gate of the second switching element Q2. The waveform diagram of FIG. 2 is based on that the low-side second secondary winding S2 is wound in the same direction as the primary winding P.
When the pulse signal PL is low, the first drive signal Hg is high to turn on the first switching element Q1 to supply power of the DC power source VDC to the load 1. When the pulse signal PL is high, the second drive signal Lg is high to turn on the second switching element Q2 to discharge energy of the load 1.
At time t10, the pulse signal generator 2 stops generation of the pulse signal PL, to stop the drive circuit. At this time, the second secondary winding S2 wound in the same direction as the primary winding P provides a negative output voltage to turn off the second switching element Q2.
On the other hand, the first secondary winding S1 that is oppositely wound to the primary winding P provides a positive output voltage. When this voltage exceeds a threshold voltage Vth of the gate of the first switching element Q1, the first switching element Q1 turns on for a period tON. Thereafter, an excitation inductance L (not illustrated) of the transformer T1 and the capacitor C cause an LC resonance. Due to the consumption of resonant energy, a voltage Vc across the capacitor C gradually decreases.
There is another related art disclosed in Japanese Unexamined Patent Application Publication No. 2002-320376 (Patent Document 1). This related art is a method of driving a power switch element. According to the related art, a switching regulator drives through a drive transformer the power switch element with a pulse signal. When stopping the pulse signal, the related art gradually decreases the duty, i.e., the pulse width of the pulse signal, or the voltage of the pulse signal, thereby protecting the power switch element from being damaged by a voltage free oscillation caused by an inductance of the drive transformer and input and output capacitors.